A plethysmogram is a measurement of the volume of an organ. A photoplethysmogram (PPG) is a plethysmogram obtained using optics. One application of PPGs is heart rate monitoring. A source of light (typically a Light-Emitting-Diode (LED)) illuminates the skin and underlying tissue and a light-intensity sensor (typically a photo-diode) measures the amount of light either transmitted or reflected. The intensity of the transmitted or reflected light corresponds to the amount of blood volume. With each cardiac cycle, the blood volume changes in the arteries and arterioles in the subcutaneous tissue, and changes in blood volume modulate the transmitted or reflected light.
One common PPG application is heart-rate monitoring during physical activity or continuous health monitoring during daily life. Wrist-bands, watches, and other wearable heart-rate monitors using PPG sensors are commonly available. One problem with wearable heart-rate monitors is that the PPG signal is also modulated by motion artifacts due to arm movement, walking, jogging, etc. Motion can move the PPG sensor relative to the skin, or change the contact force, or deform the tissue next to the sensor, or affect blood flow near the sensor (for example by partially occluding an artery), each of which may affect the PPG signal.
Reduction of motion-induced artifacts in PPG signals has been extensively investigated in industry and academia. Most approaches use accelerometers to separately measure motion. One approach uses accelerometer signals to control various types of adaptive filters. Other approaches require real-time computation of three Fast-Fourier-Transforms (FFT's) to determine three-dimensional motion frequency, which leads to high complexity, expense, and increased battery drainage.